Is it T Stops not F Stops we should care about?

Joules

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That said depending on our own personal financial circumstances buying a 600m lens whether Sigma zoom or used Canon prime can be a significant financial outlay and an emotional purchase.
Just to be sure, any kind of used Canon 600 mm lens will still not come close to a Sigma 150-600 mm C in terms of price, will it? Unless we're talking RF, in which case the 600 mm f/11 is of course already competitive on price at least.

Depending on the type of older camera in use, that might actually be the more impactful investment opportunity in terms of improving the experience.
 
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AlanF

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All of their measurements are made from RAW images, they do not use an optical bench (unlike, for example, Roger Cicala at Lens Rentals). Thus if you take a lens like the excellent Sigma 85/1.4, it is T=1.6 on a 1D X II, T=1.7 on a 5DIV, T=1.8 on a 5DsR, and T=1.9 on a 7DII.
I checked out the 400mm DO II and the 100-400mm II at 400mm on those bodies and the T-stops don't vary. But, with the Canon 85/1.4, they vary similarly to the Sigma to give a value of 1.575±0.048. It's very odd. I wouldn't be surprised if they tested the super-teles on just one of the bodies and used the same value for all.
 
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MonsMeg

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Just to be sure, any kind of used Canon 600 mm lens will still not come close to a Sigma 150-600 mm C in terms of price, will it? Unless we're talking RF, in which case the 600 mm f/11 is of course already competitive on price at least.

Depending on the type of older camera in use, that might actually be the more impactful investment opportunity in terms of improving the experience.
Good lateral thinking. Yes, that's a very fair point. She has previously mentioned the Sigma 150-600mm and possibly an R6, vs her existing 5D(mark3?) and the used Canon 600mm. It's quite possible the improvement in sensor tech / light sensitivity dominates any T stop issues. I think she was trying to frame the issue 'simply' by just focusing on the lens, but it's actually the combination of lens and camera that'll move the dial more.
 

SteveC

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I'll try to pull this all together briefly (and if I get something wrong please tell me)

The F stop is a sort of "theoretical" light transmission, if the lenses are perfectly transparent how much light is going to get through (per unit surface area of the sensor). That depends solely on the focal length and the diameter of the aperture, such that the convenient way to compare is to take the ratio, f/d.

The depth of field is purely dependent on this, so if that effect is what's important to you, it's all you need.

Glass, of course does absorb some light, and this imposes a penalty on light transmission (but not on depth of field). Two lenses of the same focal length and aperture, but with different amounts of absorption, under identical circumstances will lead to one photograph looking slightly less exposed than the other, something you can compensate for with either different exposure time (if your subject isn't moving too quickly) or ISO. It's as if you had two identical lenses, but one lens had a very light neutral density filter in front of it. This additional difference in "brightness" is accounted for in the T stop, which is basically the F stop plus this difference.

But the fact of the matter is the difference is likely to be much less than a third of a stop, so you'll only be able to completely correct for it in post processing. If it matters to you that much.

The reason it's important to cinematographers is they often cut from one camera (and lens) to another; if there's a T stop difference that's uncorrected for it could be obnoxious--it looks darker from over Kirk's shoulder looking at Spock speaking, than looking over Spock's shoulder looking at Kirk speaking; flip back and forth often enough it's distracting. They'll want to correct for it, and having a pair of numbers (T stop one is 2.6 and T stop two is 2.3 for example--which is a fairly large difference actually) lets them know there's a 0.3 stop between the two cameras and they can just brighten the 2.6 camera's stuff by 0.3 stops and call it a day.

Actually I can imagine a T stop mattering to stills photography in one circumstance. If you imagine you're going to be going off where the light is very marginal, a t-stop difference of 0.1 or so might just make the difference--sometimes--between getting a good shot and not getting one, but honestly I can't imagine that the line between good and bad is that sharply defined--will 0.1 stops of ISO ever be enough to ruin what would otherwise be an acceptable picture?
 

Joules

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The depth of field is purely dependent on this, so if that effect is what's important to you, it's all you need.
Not quite sure I agree with that if I understand it correctly. A picture taken at 24 mm f/11 will appear sharp entirely, while one taken with the 800 mm f/11 will feature just a thin slice of acceptable sharpness.

Just as a picture taken at 50 mm 1.4 will looked virtually identical to one at 100 2.8 when cropped to the same field of view and adjusted for brightness.

In all cases assuming the same focal and subject distance of course.

In any case DOF depends on how large your viewing the image, as it is defined in terms of acceptable sharpness. What may look sharp when the entire image is inspected could appear blurry at 100 % magnification.

Nonetheless, f stop is more relevant than t stop, especially since it does affect DoF. Just not exclusively.
 

SteveC

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Not quite sure I agree with that if I understand it correctly. A picture taken at 24 mm f/11 will appear sharp entirely, while one taken with the 800 mm f/11 will feature just a thin slice of acceptable sharpness.

Just as a picture taken at 50 mm 1.4 will looked virtually identical to one at 100 2.8 when cropped to the same field of view and adjusted for brightness.

In all cases assuming the same focal and subject distance of course.

In any case DOF depends on how large your viewing the image, as it is defined in terms of acceptable sharpness. What may look sharp when the entire image is inspected could appear blurry at 100 % magnification.

Nonetheless, f stop is more relevant than t stop, especially since it does affect DoF. Just not exclusively.

Fair enough. I was imagining very similar lenses. Thanks!
 

MonsMeg

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Thanks for your help everyone. I think where I'm at is :

1. My friends and I can pool a 150-600mm Sigma and 600mm Canon lens along with a common camera body, and conduct a real world test. Hold shutter speed and f stop constant and see if iso is noticeably different etc etc. In twilight conditions relevant to wildlife photographers. This will let my friend test other stuff like handling, sharpness etc too.
2. Playing with the DxO site and their T values on a proxy Canon 500mm and Sigma 150-500mm (at 500mm) that they test suggests we should observe circa 1/2 a stop 'less than advertised' on the Sigma lens (relative to the Canon).
3. Recognise that a new camera body and sensor may well offer more light gathering sensitivity among other associated benefits.

I'll endeavour to come back and let you know how this played out one way or another just FYI. My friend isn't a procrastinator so I'd expect a decision one way or another. Cheers.
 

privatebydesign

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If somebody is trying to choose between a third party 150-600 zoom and an on brand 600 prime I’d say T stop differences are the least of their concerns. They are completely different lenses suitable for very different jobs and it is far more important to work out if they need a zoom or a prime first.

Once they have answered that question, which is entirely dependent on a specific users intended use case, then start getting into the weeds on other factors of comparable lenses. If you are making a long list of pros and cons then T stop between comparable lenses would rank in the lower 20. Price, availability, service, AF accuracy, image stabilization, body compatibility, works with TC’s, image quality, size, weight, portability, and a ton of other factors would be far more important to anybody than t-stop.

But first, get them to work out if they need a zoom or a prime.
 

MonsMeg

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If somebody is trying to choose between a third party 150-600 zoom and an on brand 600 prime I’d say T stop differences are the least of their concerns. They are completely different lenses suitable for very different jobs and it is far more important to work out if they need a zoom or a prime first.

Once they have answered that question, which is entirely dependent on a specific users intended use case, then start getting into the weeds on other factors of comparable lenses. If you are making a long list of pros and cons then T stop between comparable lenses would rank in the lower 20. Price, availability, service, AF accuracy, image stabilization, body compatibility, works with TC’s, image quality, size, weight, portability, and a ton of other factors would be far more important to anybody than t-stop.

But first, get them to work out if they need a zoom or a prime.
I appreciate the post privatebydesign and agree with it. I'd probably add to your list

Lens compatibility or not with any in-body IS (if she goes with the R6 option for example).
Lens profile support or not on any post production platform (in her case Adobe Lightroom).
The relative 'sweet spots' on different lenses in terms of image quality as F Stops progress. Specifically for her how do the lenses IQ compare 'wide open' because if we're discussing T stops (which look second order...or as you suggest 20th order) you'll likely already be shooting wide open to capture low light levels (which as a wildlife photographer she overwhelmingly will be : dawn / dusk).
Use of a tripod / monopod / beanbag. Quite frankly possibly the 'best' and cheapest upgrade of all, and the lowest tech. Within that should IS be switched on or off when using a tripod (arguably off), and within that do you need to bother because the lens has tripod sensing IS software.
There'll be other factors too no doubt.

So why do I care about T stops again, if we're both aware of this other stuff?

1. My friend asked me to investigate (just) them quantitatively, plus I'd not come across them before. So I wanted to find out how important (or not) they were, and the DxO site with AlanF showing me that the Transmission data comes as a graph for an extended zoom range gives the granularity at different focal lengths I need.
2. She's investigated (ing) the other stuff by herself and with others. I'm of use because I use a Canon 600mm f4 IS ii for example, and I was happy to look at T stops on her behalf, the other friend has the Sigma 150-600mm and she'll no doubt call on others if they can help too (she's a pretty smart cookie and fairly gregarious).
3. I posted a specific post on T stops to try and keep the discussion narrow and focused (so I could get an answer), and to draw in those with the specific knowledge in that area.

So a big thanks once again to everyone, I think the DxO graphs specifically give us the required granularity to put T stops in their proper place in order of importance.
 

MonsMeg

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I thought I'd post the outcome of a little real-world field test we conducted. Mainly because the outcome was a little more interesting / unexpected than the test-bench / theoretical discussions up to that point suggested. Right up front if you're not into wildlife shooting at golden hour with long lenses, you can jog-on and correctly state that T stops don't matter and at best/worst cost you 1/3 stop.

What did we 'discover'? Well it looks like you can lose around 0.7 to maybe a 1 stop of light over the advertised F Stop on your lens when using a long zoom. In other metrics that would mean having to drop from 1/1000 sec to 1/500sec, or using a higher iso, or more interestingly for us walking away from golden hour maybe 20mins early (than using a prime at the same F stop and with the same camera).

Kit? We took a couple of cameras (to try different chips 1Dxiii and 5Diii) and four lenses, a newish Canon 600mm F4 is ii, a couple of Sigma 150-600mm f5.6-6.3 lenses (contemporary and sport), and an old Canon non IS 600mm f4. What did we do? Set everything to f6.3 and 1/1000 sec and watched what happened to iso on the camera displays over an hour or so as the sun set. The Canon's both performed "the same" as did the two Sigma's. The Canon's at f6.3 would chuck out an iso of 800 on the 1Dxiii and the Sigmas an iso of 1250 / 1600. From memory it was a similar differential on the 5Diii. We also played about with the DoF preview button just to make sure the Canon's weren't somehow sneaking in a bit more light with that f4 aperture.

If you arbitrarily set parameters after which you'd stop shooting, blurry images, too grainy etc. So for us we chose f6.3, 1/500sec, iso1600 (mainly because of the prevailing light) then in our part of the world you'd have walked away 20minutes earlier with the zooms vs the primes if everyone had an f6.3 aperture. Or put another way, if golden hour is an hour, you lose 1/3 of it.

Will my friend who started this ball rolling now go for the prime? Haha, actually no, she's going with the Sigma. Our little test allowed her to handle a range of different lenses all 'together' and explore those 'other factors'. T stop was more important than we were all expecting though.

For me I've probably come away liking my primes a little more, and probably (for my style of shooting) agreeing with DxO on the relative importance of light transmission as a factor for deciding between (long) lenses.

As a disclaimer I should say we don't plan on re-running this test, it's fairly time-consuming to organise 4 people with multiple lenses etc in the real world and we're not paid to do this. That said if we've obviously 'missed a bit' (that is likely to be material) please jump in. Certainly I think I learnt something new anyway, if we didn't jog the tripod when changing cameras, if a cloud didn't appear and lower ambient light levels as we changed camera, if..... All the best. ;)
 
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jd7

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Awww. You're too kind privatebydesign, thank you sooo much....:)
While I agree with PBD that you would expect a $12k lens to be better than a $900 one, and with camera gear (and many other things) often you do get what you pay for, there is no guarantee things will be as you expect so never a bad thing to check if you can (you know what they say about assumption!). And in any event still useful (at least for me) to hear about someone's attempt to directly compare lenses in practical terms (as against differences in specs / theoretical performance). Thanks for posting.
 

Czardoom

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Hmm, so a $12,000 lens is better than a $899 lens even at the same aperture, who'd have thought? Great work...
A person goes to a certain amount of trouble to test out the T-stops of various lenses in a real world test, and all you can come up with is a smart-aleck arrogant response? Great work...
 
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privatebydesign

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A person goes to a certain amount of trouble to test out the T-stops of various lenses in a real world test, and all you can come up with is a smart-aleck arrogant response? Great work...
:ROFLMAO: If that’s what you think happened here....... Great work....
 
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While I agree with PBD that you would expect a $12k lens to be better than a $900 one, and with camera gear (and many other things) often you do get what you pay for, there is no guarantee things will be as you expect so never a bad thing to check if you can (you know what they say about assumption!). And in any event still useful (at least for me) to hear about someone's attempt to directly compare lenses in practical terms (as against differences in specs / theoretical performance). Thanks for posting.
100% I still found it a very interesting test and appreciate the post by MonsMeg.

Some people care about the last 10-20% performance gains even if it results in a 10x higher cost. Spend some time in the world of high end audio it is very common for people to spend a crazy amount of money for less than a 10% difference in sound quality… It’s the same with cameras a lot of the time
 
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